The invention relates to the fuel mixture control for internal combustion engines. More particularly, the invention relates to a fuel mixture control system which includes an oxygen sensor for monitoring the composition of the exhaust gas and for generating a signal which is used to control the fuel-air ratio in the combustible mixture fed to the engine. In known fuel supply systems of this kind, there may be provided for example a carburetor or an electrical fuel injection system which is subject to regulation by the signals from a so-called .lambda. or oxygen sensor disposed in the exhaust manifold. The .lambda.-sensor signal is processed in appropriate electrical circuitry to produce a control signal which is useable by the fuel injection system or more generally by the fuel preparation system. In commonly used oxygen sensors, the output signal is a step function which changes its value abruptly at the point where the mixture is approximately stoichiometric. However, it is well known that the output signals from the oxygen sensors are not useable in the domain of relatively low temperature, in particular the normal temperature of a cold, i.e. stopped engine. Thus it is generally required to control the engine in some other way until it has reached a suitable elevated temperature at which time the .lambda.-sensor signal is used. The reason for the diminished utility of the .lambda.-sensor signal is the very high internal resistance of the sensor at lower temperatures. Furthermore, in the critical temperature region where the internal resistance of the sensor is high, the output signal is also highly non-linear and for these reasons it is generally preferred in known systems to switch the mixture control to a forward open-loop manner of control and to close the control loop only after the .lambda.-sensor generates clear and useable output signals.